1. Field of the Invention
The present invention relates to the processes of racemization and deprotection of special N-protected amino acids in the acylase/racemase system for the total conversion of special N-protected racemic amino acids into optically pure amino acids.
2. Discussion of Background
Optically pure enantiomers of both L- and D-amino acids are important starting compounds in chemical syntheses, as well as for parenteral nutrition. Many methods of producing oprtically pure amino acids are possible and known to a person skilled in the art. Suitable processes include relevant enzymatic processes since they take place catalytically and produce with very high enantiomer concentrations of amino acids.
A racemic mixture of amino acids is not optically pure and contains both L-amino acid and D-amino acid enantiomers. Both L-amino acid and D-amino acid enantiomers can be utilized through enzymatic catalysis of a racemic mixture of N-protected amino acids. For example, it is known that L-amino acids are prepared from a racemic mixture of N-acetylated amino acids by using amino acid acylases. However, it is thought that these acylases are specific only for the cleavage of N-acetyl-protected amino acids and amines/alcohols (EP99118844.2; A. S. Bommarius et al., Tetrahedron; Asymmetry, 1997, Vol. 8, 3197–3200). Further, various racemization processes have been developed to prepare L-amino acids from the remaining D-acetyl amino acid fraction of the racemic mixture. DE 19935268.2 discloses an acetylamino acid racemase in the acylase/acetylamino acid racemase system that can prepare optically pure L-methionine from a racemic mixture of N-acetylmethionine.
Less specific N-acetylamino acid racemases (AAR) have been described previously. These racemases can be found in the microorganisms Streptomyces atratus Y-53 (Tokuyama et al., Appl. Microbiol. Biotechnol. 1994, 40, 835–840) and Amycolatopis sp. TS-1-60 (Tokuyama et al., Appl. Microbiol. Biotechnol. 1995a, 42, 853–859). For example, the racemase of Amycolatopis sp. TS-1-60 can catalyze the racemization of N-carbamoylamino acids to L-amino acids, although with less than optimal activity.
Processes for complete conversion of amino acids other than acetyl-protected or carbamoyl-protected amino acids to optically enriched amino acids are not known. Further, racemases with the ability to convert amino acids other than acetyl-protected or carbamoyl-protected amino acids are not known. Therefore, there is a need for enzymatic processes that racemize N-protected amino acids in general, as well as those N-protected amino acids other than acetyl-protected or carbamoyl-protected amino acids. The N-protected amino acid products of such racemic converstions may then be converted into the optically enriched amino acid by a subsequent enzymatic cleavage of the protecting group(s).